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Numerical value, notation, and units

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The speed of light in vacuum is usually denoted by a lowercase c , for "constant" or the Latin celeritas (meaning "swiftness, celerity"). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √ 2 times the speed of light in vacuum. Historically, the symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c , which by then had become the standard symbol for the speed of light. Sometimes c is used for the speed of waves in any material medium, and c 0 for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as other related constants: namely, μ 0 for the vacuum permeability or magnetic constant, ε 0 for the v
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Speed of light

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The speed of light in vacuum, commonly denoted c , is a universal physical constant important in many areas of physics. Its exact value is defined as 299 792 458  metres per second (approximately 300 000  km/s, or 186 000  mi/s).Note It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of ​ 1⁄ 299 792 458 second.Note According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any information can travel through coordinate space. Though this speed is most commonly associated with light, it is also the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation (of which light is a small range in the frequency spectrum) and gravitational waves. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonze
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Faster-than-light observations and experiments

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There are situations in which it may seem that matter, energy, or information travels at speeds greater than  c , but they do not. For example, as is discussed in the propagation of light in a medium section below, many wave velocities can exceed  c . For example, the phase velocity of X-rays through most glasses can routinely exceed c , but phase velocity does not determine the velocity at which waves convey information. If a laser beam is swept quickly across a distant object, the spot of light can move faster than  c , although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed  c . However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed  c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than  c , after a delay in time. In neither case does any matter, energy, or informat
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